New here, Quick question

[hoodmiser2004]

I've been trying to jet my kfx400, I've tried every main jet size you can think of and i cannot get it to stop backfiring at wide open throttle. Someone over on z400C was saying my ignition coil might be lose? Any ideas.

[Vindex Injuriae]

This might be a long shot, but I was having similar probs with mine. Turns out, the diapragm for the slide, where the needle goes through, was pinched in between the cover and the carb. The dealer had changed the jetting prior to me buying it and pinched the diaphragm when he put it back together. I fixed it, corrected the jetting and have had no probs since then.

Like I said, unusual and a remote possibility, but it happened to me....

[indyZrider]

if your reving it while its sitting in the garage and your not riding it you are probably bouncing it off of the rev limiter and you are mistaking that for the backfiring. its an easy and honest mistake to make. with your mods you should probably be around a 155-160 dj if you have the lid off. if your lid is still on then you will be around the 146-150 area. you will need to adjust to air/fuel mix screw as well. and almost every one has to bump up the pilot jet from a 22.5 to a 25 when you start modding. just a slip-on doesnt change the need for a bigger jet all that much so since stock is 130 which is a 140 dj then at a 150 dj your only a little higher than stock. if your up in the 170's then youre way too high. also there is a size difference between the brands of jets. mikuni is bigger than dyno-jet. so the numbers will be different. good luck